Hamilton complained that Reid did not succeed in being a very good Natural Realist, and that he slipped unconsciously into the position he was concerned to condemn.  Sir William tried to eliminate this error, but the careful reader of his works will find to his amusement that this learned author gets his feet upon the same slippery descent.  And much the same thing may be said of the doctrine of Herbert Spencer (1820-1903), who claims that, when we have a sensation, we know directly that there is an external thing, and then manages to sublimate that external thing into an Unknowable, which we not only do not know directly, but even do not know at all.

All of these men were anxious to avoid what they regarded as the perils of Idealism, and yet they seem quite unable to retain a foothold upon the position which they consider the safer one.

Reid called his doctrine the philosophy of “Common Sense,” and he thought he was coming back from the subtleties of the metaphysicians to the standpoint of the plain man.  That he should fall into difficulties and inconsistencies is by no means surprising.  As we have seen (section 12), the thought of the plain man is far from clear.  He certainly believes that we perceive an external world of things, and the inconsistent way in which Descartes and Locke appeal from ideas to the things themselves does not strike him as unnatural.  Why should not a man test his ideas by turning to things and comparing the former with the latter?  On the other hand, he knows that to perceive things we must have sense organs and sensations, and he cannot quarrel with the psychologists for saying that we know things only in so far as they are revealed to us through our sensations.  How does he reconcile these two positions?  He does not reconcile them.  He accepts them as they stand.

Reid and various other philosophers have tried to come back to “Common Sense” and to stay there.  Now, it is a good position to come back to for the purpose of starting out again.  The experience of the plain man, the truths which he recognizes as truths, these are not things to be despised.  Many a man whose mind has been, as Berkeley expresses it, “debauched by learning,” has gotten away from them to his detriment, and has said very unreasonable things.  But “Common Sense” cannot be the ultimate refuge of the philosopher; it can only serve him as material for investigation.  The scholar whose thought is as vague and inconsistent as that of the plain man has little profit in the fact that the apparatus of his learning has made it possible for him to be ponderously and unintelligibly vague and inconsistent.

Hence, we may have the utmost sympathy with Reid’s protest against the doctrine of representative perception, and we may, nevertheless, complain that he has done little to explain how it is that we directly know external things and yet cannot be said to know things except in so far as we have sensations or ideas.